Test results for a strain of Strep A which has killed five people in Christchurch confirm it was a highly invasive strain of the disease.
The strain killed five residents of Bishop Selwyn Lifecare resthome earlier this month.
Strep A is a common cause of most illnesses such as sore throats or coughing, but the test results confirm suspicions this was a particularly aggressive strain of the infection.
Canterbury's Medical Officer of Health, Ramon Pink, said that's what caused the infection to spread so rapidly among such a small group of people.
Dr Pink said when coupled with conditions resthome residents may already have, such as diabetes or heart conditions, aggressive strains of Strep A can kill.
He said he's confident the Bishop Selwyn resthome has good practices in place, and there is no further danger to residents or the community.